1. Field of the Invention
The present invention relates to a state detection device, particularly, a state detection device that detects driving states of a plurality of loads connected with each other in parallel and driven by a driving current supplied by a driving unit.
2. Description of the Related Art
In recent years and continuing, liquid crystal displays are attracting attention as display devices of computers or televisions, because they can be made thin and have low power consumption. The liquid crystal displays include transmission type liquid crystal displays and reflection type liquid crystal displays, and in each of the transmission type liquid crystal displays, a back light is provided to supply light from the back side of the screen of the liquid crystal display.
In many cases, electric discharge tubes, such as CCFLs (Cold Cathode Fluorescent Lamps), are used in the back light. The number of the CCFLs is increased depending on the size of the liquid crystal display. For example, usually, four CCFLs are used for a 17-inch liquid crystal display.
FIG. 4 is a schematic view showing a system having four CCFLs.
As illustrated in FIG. 4, there are four CCFLs 101-1 through 101-4. In order to simplify electrical connections, the CCFLs 101-1 through 101-4 are divided into a pair of CCFLs 101-1 and 101-2, and a pair of CCFLs 101-3 and 101-4, and the two pairs are respectively connected to three-pin connectors CN1 and CN2.
The connector CN1 includes terminals T11, T12, T13, and the connector CN2 includes terminals T21, T22, T23.
As illustrated in FIG. 4, the high voltage end of the CCFL 101-1 is connected to the terminal T11, the high voltage end of the CCFL 101-2 is connected to the terminal T12, and both the low voltage end of the CCFL 101-1 and the low voltage end of the CCFL 101-2 are connected to the terminal T13. The high voltage end of the CCFL 101-3 is connected to the terminal T21, the high voltage end of the CCFL 101-4 is connected to the terminal T22, and both the low voltage end of the CCFL 101-3 and the low voltage end of the CCFL 101-4 are connected to the terminal T23.
The terminals T11, T12 and the terminals T21, T22 are connected to the same connection point through condensers C, and are further connected to a driving circuit or others. The terminals T13 and T23 are grounded through resistances.
Usually, the CCFL, which is an electric discharge tube, is turned on by a high voltage, and after being turned on, the ON state of the CCFL is maintained by a low voltage. If the turn-on operation fails, the CCFL has to be lit on once again. For this reason, in the related art, it is necessary to detect the ON state of the electric discharge tube. In the related art, a circuit is used to detect the voltages on the two ends of the electric discharge tube to determine the ON state thereof. This technique is disclosed in, for example, Japanese Laid Open Patent Application No. 7-45379, Japanese Laid Open Patent Application No. 11-67474, and Japanese Laid Open Patent Application No. 2000-21586.
However, the circuit of the related art can just detect the ON or OFF state of one electric discharge tube.
In the case of plural electric discharge tubes, such as a back light of a liquid crystal display, even when only one of the electric discharge tubes is turned on, the detection circuit of the related art determines that the back light is in an ON state. But, in the back light of a large liquid crystal display using plural electric discharge tubes, if one of the electric discharge tubes fails to be turned on, the other electric discharge tubes are turned on, the brightness of the liquid crystal display screen ends up being non-uniform, and the liquid crystal display cannot operate appropriately. For this reason, it is required that a detection circuit be able to reliably detect failure of lighting on even one electric discharge tube.